This invention relates to an optical integrated circuit for heterodyne detection which can be used in coherent optical communications, optically applied measurements, etc.
The modulation and demodulation system of optical communications depends, at present, mainly on direct modulation and direct detection. There is, however, a great demand for a larger information transmission capacity and an extension in the distance between repeater intervals. As such, the realization of coherent optical communications is expected. In coherent optical communications, the transmitted light signal is received by heterodyne or homodyne detection at the receiving end. The structure of the optical heterodyne or homodyne receiver is shown in FIG. 1. A light signal 1 is synthesized with a local oscillation light 3 emitted from a local oscillator 2 at a beam splitter 4, and is detected by a photo detector 5. Numeral 6 denotes an automatic frequency control system or loop filter. In heterodyne detection, in order to keep the intermediate frequency at a constant value, or in homodyne detection, in order to synthesize the phase of the local oscillation light with the phase of light signal, the local oscillator 2 is controlled by a detection signal 7.
In this system, when a local oscillator with sufficiently large power is used at the receiving end, a reception sensitivity at shot noise limit may be obtained, and a sensitivity improvement of 10 to 30 dB may be expected as compared with the direct detection system.
To realize the coherent optical communications, however, there are many technical problems to be solved. In particular, the improvement of the spectral linewidth of the semiconductor laser is an important issue.
The spectral linewidth can be improved by composing the semiconductor laser in an external cavity structure as reported by Wyatt, R. et al. in Electron. Lett. 19, pp. 110-112, 1983, but it tends to be mechanically unstable in the case of hybrid composition. However, as we reported in Electron. Lett. 21, pp. 374.gtoreq.376, a stable narrow spectral linewidth is obtained when the external cavity is integrated monolithically.
Since the receiver shown in FIG. 1 is not monolithic, the entire apparatus is very large in size.